Often it is necessary to cool a working fluid, and it is known for this purpose to use a heat exchanger. Heat exchangers often comprise one or more metallic tubes suspended between two tube plates. The working fluid to be cooled, which may for example be water or oil, flows through the tubes, whilst the coolant passes around and between those tubes, the working fluid giving up its latent heat to the tubes and thus to the coolant.
The effective surface area of a tube can be enlarged in order to increase the heat transfer, as by the addition of one or more annular extended surface members or fins in thermal contact with the outer surface of the tube. Such finned tubes are particularly useful if the coolant has a low viscosity, and if the coolant is a gas, such as air.
If the tubes are to withstand the internal pressure of the fluid to be cooled, the addition of the fins should not reduce or significantly reduce the tube bursting strength. If the fins are to increase the heat transfer they should not significantly inhibit the flow of coolant, and preferably should encourage turbulent coolant flow.
The tubes to be used for heat exchangers should meet certain standards (in the UK for instance British Standard 2871 Part 3), these standards being relevant also for those tubes which are formed by extrusion to provide selected internal formations chosen to enhance internal turbulent flow i.e. to avoid laminar or stratified flow of the working fluid to be cooled; desirably the tube finning should not reduce those respective standards e.g. of tube wall thickness and thus of strength, or of tube uniformity and fin engagement and thus of heat transfer to the fins.
The fins should be positioned on the tube so as to encourage maximum heat transfer to the coolant, which will not occur if the fin spacing is irregular, or if the fin angles are irregular (with an annular gap of varying axial length between adjacent fins).
If the tube walls need to be thinned to accept the fins, one or more of the tubes may burst in service and need to be plugged; if the fins are irregularly spaced and/or angled the performance of the heat exchanger will be reduced.
It is a known design criterion when constructing a matrix or array of finned tubes for a heat exchanger both to arrange the tubes as close to each other as possible (to reduce the size of the heat exchanger), and to have a maximum area available for thermal transfer between the working fluid and the coolant (to maximise the possible heat exchange). When utilising tubes fitted with the known annular fins in such an array, the spacing between the tubes will be limited by the outer diameter of the fin(s); if as is usual the fins have circular outer peripheries there are areas between adjacent tubes which do not contribute to heat transfer, and a finning method and machine permitting fins to be fitted which can utilise these areas is desirable.
The performance of a heat exchanger in part depends upon the number of fins fitted to a tube, and to the total number of fins i.e. to the aggregate extended area available for heat exchange, as well as to the positioning and disposition of those fins.
When seeking to overcome the disadvantage of the unused heat-exchange area caused by the use of fins with a circular periphery, it is known to replace the separate fins of adjacent tubes by axially-spaced “common-fins” i.e. fins which engage (and interconnect) several tubes. Typically, a common-fin takes the form of an extended plate having several apertures, each aperture being adapted to receive a respective tube, the plate-like common-fin being in simultaneous thermal contact with several tubes, and being adapted to transfer the heat from all of the tubes across the full area therebetween. An array of tubes to which are mounted a plurality of multi-apertured common-fins is referred to herein as a “fin block”, though in other documents it is also referred to as a “coil block” or “block fin”.
It will be understood that in a fin block, each fin can be continuous between and around each tube in the block, so that a minimum of heat transfer area is wasted. It will be further understood that the tubes in each block are fixed relative to the other tubes of that block by the prefitted plate-like common-fins.
A known further advantage of such assembled fin block is its relative ease of fitment into a heat exchanger. Thus, for a heat exchanger requiring two hundred separate finned tubes for instance, each of the two hundred tubes must be fitted to both tube plates, and perhaps also to separate support plates as may be required for longer tubes. However, if a fin block is prepared having twenty tubes, then only ten such blocks are required to be handled and fitted.
In a known method of manufacturing a fin block, a stack of common-fins is arranged, adjacent fins being axially spaced by a distance to suit the requirements of a particular heat exchanger; each common-fin has several apertures, the apertures corresponding in pattern to the required tube arrangement. The apertures are slightly larger than the outside diameter of the tubes, and the common-fins are held with their respective apertures aligned. The tubes are then individually passed through the apertures, and when in position a “bullet” is pulled through each tube, to expand the tube wall into mechanical contact with the respective fin apertures. A method of this general type is disclosed in U.S. Pat. No. 3,889,745.
This method is not suitable for extruded tubes having internal formations (e.g. for promoting turbulent flow of the liquid to be cooled), since a bullet cannot be passed through such tubes.
Another disadvantage of this known bulleting method is that the wall thickness of the tubes is limited by the need for the wall to be stretched by the bulleting operation, so that thinner-walled tubes have to be used than might otherwise be desired; for example, in practice for a stainless steel tube with an outer diameter of 0.75″ (19.05 mm) it is rare for tube thicker that 22 Gauge (“Standard Wire Gauge”) to be bulleted. A further disadvantage is that the bulleting operation introduces stress into the tubes, and can change the grain structure; the stress is typically not removed by heat treatment since the heat treatment would act also to soften the fins and reduce the thermal contact between the fins and tubes, i.e. the stress induced by the bulleting operation typically remains in the tube and thus in the heat exchanger as an unwanted side effect of this method of production. Yet another disadvantage is that the material specification of the tubes can be altered by the bulleting operation; for example, if the heat exchanger user specified that annealed tubes should be used, the bulleting operation can in some circumstances alter the annealed material into a non-annealed hardened state. Another further disadvantage is that the tube must be of deformable material, so limiting the material which can be used.
Bulleting can also result in non-parallel finning. As the bullet is pulled through the tube, the tube wall can form an angled “front” which moves down the tube immediately ahead of the bullet, as a “ripple”; adjacent fins subject to the “ripple” are likely either to be moved along the tube, or to adopt different angles relative to the tube, resulting on occasion with parts of adjacent fins touching and with other parts spaced by a greater distance than intended. The expansion caused by the bullet is such that once the bullet has passed a fin, the position of the fin cannot subsequently be corrected or altered.
Usually when “rippling” is seen to be occurring during tube finning the bullet has nevertheless to be fully drawn through the tube so that it can be reused, even though the manufacturer recognises that the resulting finned tube is likely to be rejected as unsuitable for heat exchanger use. Also, if a set of tubes is finned whilst in position in a heat exchanger array, any fin displacement which occurs upon internal and thus non-visible tubes cannot be observed, so that the resulting loss of heat exchanger performance might not be realised until the exchanger is in service.
U.S. Pat. No. 3,733,673 discloses a machine for fitting several fins to one or two tubes at the same time. The fins are arranged in a cartridge, and held along their top and bottom edges. Each fin has a number of apertures therein which are sized and shaped to correspond closely with the outer periphery of the tubes to be fitted thereinto. The machine is pneumatically actuated and can drive one or two tubes at a time though the aligned apertures in the fins. Following insertion of the fin or fins, the machine can subsequently be used to insert one or two further tubes into respective apertures of the fins, until all of the tubes have been inserted thereinto.
It is a disadvantage of this machine that the cartridge provides only limited support for the fins, and so the fins need to be sufficiently rigid to remain undeformed by the insertion of the tubes. On the other hand, it is desirable that there be sufficient engagement between the tubes and the fins to ensure good heat transfer therebetween, and it is also desirable that the fins be as thin as possible to reduce the weight of the assembled fin block and also to maximise heat transfer. A compromise is therefore necessary between reducing the thickness of the fins so as to maximise heat exchange, and increasing the thickness of the fins so as to ensure that the fins do not become deformed as the tubes are forced therethrough. A further compromise is necessary between arranging the fins to be a tight fit upon the tubes (so as to maximise the heat transfer therebetween), and arranging the fins to be a loose fit upon the tubes so that the tubes can be moved more easily therethrough with a reduced likelihood of deformation of the fins.
It is another disadvantage of this machine that only two tubes can be inserted into the fins at a time, so that considerable time is taken to produce a large fin block. The provision of a maximum of two tubes is due to some extent to the maximum power of the machine, but greater relevance is understood to be given to the practical disadvantage that as more tubes are inserted at the same time, the greater is the likelihood of deforming some of the fins.
A machine for making fin block (though it could also apply individual fins to individual tubes) is disclosed in WO96/35093. That machine utilises a linear motor to drive a fin (or common-fin) onto one or more tubes. In an alternative method of using the machine, a first fin can be driven onto the end of the tubes, and a second fin driven into engagement therewith, both fins subsequently being driven along the tubes to their predetermined positions. It is accepted that the collars of the two fins will interlock in such circumstances, and it is necessary that the machine have sufficient power to drive such interlocked fins.